1. Field of the Invention
This invention relates to a thermal head apparatus wherein a board, a heat radiating member and a platen roller constitute a unit, and a method of assembling a thermal head apparatus wherein the shaft of the platen roller is supported by bearings.
2. Description of the Related Art
Thermal head apparatuses are widely used in printers connected to computers. In a type of thermal head apparatus, a board, a heat radiating member, and a platen roller constitute a unit. The thermal head apparatus of this type has the following structure.
The heat radiating member is made of, for example, aluminum. The board made of, for example, ceramic is adhered to the upper surface of the heat radiating member. A heater is formed on the board. The platen roller is arranged parallel with the board and in contact with the heater. The platen roller is supported by a roller shaft which passes through the center portion thereof, and both end portions of the roller shaft protrude from the ends of the platen roller.
The end portions of the roller shaft are rotatably supported by bearings which are fixed to suitable members.
The roller shaft is rotated by a rotational-driving apparatus. A recording paper sheet is transferred between the platen roller and the heater by means of rotation of the platen roller. Electric power and a printing signal are supplied from a printed circuit board to the heater, thereby thermal-printing an image on the recording paper sheet. Heat generated by the heater is radiated through the heat radiating member.
In the above thermal head apparatus, to perform accurate thermal recording, it is necessary for the platen roller to be in contact with the heater provided in the board at the position precisely above the heater. In other words, if the platen roller is not positioned at a right position with respect to the heater, the heat energy generated by the heater cannot be applied satisfactorily to the recording paper sheet. As a result, color development on a recording portion of the paper sheet may be irregular or color is not developed at all, thus rendering a recorded image unclear. Hence, it is necessary to position the platen roller exactly above the heater.
In a conventional thermal head apparatus, positioning members are provided on both end portions of the heat radiating member to position the platen roller. The bearings for supporting the ends of the roller shaft are positioned by the positioning member pair. As a result, the platen roller is positioned with respect to the heater.
More specifically, each of the positioning members has a pair of raised portions on both sides of a point A where the platen roller is in contact with the heater, i.e., the sheet-entrance side and the sheet-exit side. The bearings are engaged with the raised portions and fixed to suitable members as described above. When the platen roller is rotated to transfer a recording paper sheet, the outer surfaces of the bearings are brought into contact with the raised portions of the positioning members provided on the sheet-entrance side with respect to the point A. This is because friction occurs between the platen roller and the recording paper and reacts against the rotational force of the platen roller transferring the recording paper sheet, thereby biasing the platen roller in the reverse direction. In this manner, the platen roller is positioned with respect to the point A with reference to a point B where the bearing is brought into contact with a positioning member.
In the structure as described above, it is necessary to accurately set a distance S between the point A where the platen roller touches the heater and the point B where the bearing touches the positioning member. To set the distance S, the following operation is performed.
First, an end of the heat radiating member is set against a jig. The board is positioned with reference to a plane C, an end of the heat radiating member, and attached thereto. Then, the positioning member is positioned with reference to the plane C and attached to the heat radiating member by a bolt. Each bearing is engaged with the corresponding end portion of the roller shaft, and inserted between the raised portions of the positioning member.
The above-described structure has the following drawbacks.
Setting of the distance S between point A and point B, includes three steps, i.e., attaching the board to the heat radiating member, attaching the positioning member to the heat radiating member, and engaging the bearings of the platen roller with the positioning members.
Attaching the board to the heat radiating member and attaching the positioning member to the heat radiating member are inevitably accompanied by assembling errors in the positions of the board and the positioning member. The distance S between the points A and B is affected by these errors in a multiplicational manner.
In summary, an error in each position greatly influences the distance S. Therefore, it is very time-consuming to set a distance S within a predetermined accuracy without an influence of errors in the positions of the board and the positioning member. Moreover, a complicated assembling jig is used to set a distance S, taken the influence of the errors into account.